The Third Generation Partnership Project (3GPP) defines a long-term evolution (LTE) architecture, which provides high data rate, low latency, packet optimization and improved system capacity and coverage. In an LTE system, an evolved universal terrestrial radio access network (E-UTRAN) includes a plurality of evolved Node-Bs (eNBs) and communicates with a plurality of mobile stations, also referred to as user equipments (UEs).
LTE technology supports packet based services only. However fallback is specified for circuit switched (CS) services as well. CS Fallback (CSFB) defines a mechanism for using a CS legacy network (e.g. the Global System for Mobile Communications (GSM)/Wideband Code Division Multiple Access (WCDMA)) to provide voice and traditional CS-domain services (e.g. voice call, a CS short message service (SMS), Unstructured Supplementary Service Data (USSD), among others). To provide these CS services, a UE may reuse the CS infrastructure when the UE is served by E-UTRAN.
When a CSFB capable UE selects an E-UTRAN, there is a combined Tracking Area Update/Location Area Update (TAU/LAU) with an international mobile subscriber identity (IMSI) attach procedure initiated. Further, when a CSFB capable UE camping on an E-UTRAN selects a tracking area that is different from one stored in it, there is a combined TA/LA update. Further, when a timer for periodic TA/LA expires, there is a combined TA/LA updating. Such procedures may result in mobile originated (MO) call failure.